Problem: Given the equation: $ y = -x^2 - 6x - 13$ Find the parabola's vertex.
When the equation is rewritten in vertex form like this, the vertex is the point $({h}, {k})$ $ y = A(x - {h})^2 + {k} $ We can rewrite the equation in vertex form by completing the square. First, move the constant term to the left side of the equation: $ \begin{eqnarray} y &=& -x^2 - 6x - 13 \\ \\ y + 13 &=& -x^2 - 6x \end{eqnarray} $ Next, we can factor out a $-1$ from the right side: $ y + 13 = -1(x^2 + 6x) $ We can complete the square by taking half of the coefficient of our $x$ term, squaring it, and adding it to both sides of the equation. The coefficient of our $x$ term is $6$ , so half of it would be $3$ , and squaring that gives us ${9}$ . Because we're adding the $9$ inside the parentheses on the right where it's being multiplied by $-1$ , we need to add ${-9}$ to the left side to make sure we're adding the same thing to both sides. $ \begin{eqnarray} y + 13 &=& -1(x^2 + 6x) \\ \\ y + 13 + {-9} &=& -1(x^2 + 6x + {9}) \\ \\ y + 4 &=& -1(x^2 + 6x + 9) \end{eqnarray} $ Now we can rewrite the expression in parentheses as a squared term: $ y + 4 = -1(x + 3)^2 $ Move the constant term to the right side of the equation. Now the equation is in vertex form: $ y = -1(x + 3)^2 - 4 $ Now that the equation is written in vertex form, the vertex is the point $({h}, {k})$ $ y = A(x - {h})^2 + {k} $ $ y = -1(x - {(-3)})^2 + {(-4)} $ The vertex is $({-3}, {-4})$. Be sure to pay attention to the signs when interpreting an equation in vertex form.